The present invention relates generally to an optical rain sensor for detecting water on an automotive window, and more particularly, to a rain sensor that is capable of operating efficiently when mounted on glass of varying thickness and composition.
In recent years, it has been increasingly common for motor vehicles to incorporate optical rain sensing wiper control systems that adjust the speed of the wipers in response to the accumulation of water on the outside surface of, for example, the windshield. The rain sensors of such systems typically employ beams of light directed through the windshield at an angle of 45 degrees. The presence of rain or snow on the outside surface of the windshield disrupts the beams, and the optical rain sensor uses that effect to determine an appropriate speed for the vehicle wipers. A practical implementation of such a system was taught by Teder in U.S. Pat. No. 5,059,877, and the teachings thereof are incorporated herein by reference.
An important factor in the success of a commercial rain sensor is the optical configuration of the sensor. Specifically, the sensor should efficiently couple rays into a vehicle window, and should yield a large sensed area. The sensor should require few opto-electronic devices in order to implement the required sensed area, and to keep the size small and the cost low. Additionally, it is desirable that the rain sensor be compatible with the windows of passenger cars, as well as the windows of larger trucks, recreational vehicles, and other specialty vehicles. Such windows are constructed with a variety of thicknesses and constructions, resulting in different infrared transmittances of the subject structures.
The rain sensor taught in U.S. Pat. No. 5,898,183 to Teder shows that a rain sensor may be made in a very compact and inexpensive form, and yet still operate in a highly efficient manner. The rain sensor of the '183 patent features two emitters and two detectors, each mounted on a planar circuit board, and couples high obliquity rays into the windshield. A consequence of the approach of the '183 patent is that the design must be nominally optimized for each different glass thickness on which the sensor is mounted. By using a sufficiently large aperture, good performance of the rain sensor of the '183 patent can be achieved for the range of thicknesses typically used in passenger cars, i.e., between 4 to 6 mm thick. But, if the same configuration is to be used for windshields deployed on, for example, recreational vehicles, which are typically 8 mm thick, the optical design, must be made proportionately larger. This makes for a physically larger rain sensor. A larger rain sensor is more costly, because of the need for more materials, and is less aesthetically pleasing.
A rain sensor for thick windshields may be made more compact if reflecting surfaces are used to fold the beams toward the inside surface of the rain sensor. Such an approach is taught by Stanton in U.S. Pat. No. 5,414,257. This approach is particularly advantageous for thick windshields, where the thickness of the windshield would dictate that the optical devices would be too far apart if implemented with the refractive approach of the '183 patent. However, the approach is less suitable when deployed on a thin windshield, as an optimal design places the emitter and detector so close together as to risk them interfering with one another. The rain sensor of the '257 patent is thus suitable for only a modest range of windshield thicknesses. A common aspect of the aforementioned rain sensors is that they operate in a single mode of operation. That is, the rays from the emitter strike an optical element, are coupled into the glass, deflect off the outside surface, and so on. Optical systems wherein all rays of interest progress from each surface to the next are known in the field of optical engineering as “sequential”, or “deterministic.” The other major class of optical systems is known as “nonsequential”. In nonsequential systems, a ray emanating from a surface may subsequently strike any of several surfaces, depending on the position and direction of the ray. All of the aforementioned rain sensors utilize sequential optical systems. The rays follow this same deterministic sequence, or mode of operation, regardless of whether the sensor is deployed on thin glass or thick. By the nature of this approach, the rain sensor must be optimized for a particular thickness of windshield. If the sensor is placed on a windshield that is much thicker or much thinner than the design optimum, the rays from the emitter do not strike the optical structure that is supposed to guide the rays to the detector. The result is that the rain sensor either functions poorly or ceases to work at all. It would be better if a sensor could operate in a different fashion for different material thicknesses.
U.S. Pat. No. 6,232,603 describes a device for detecting the presence of moisture on an outside surface of a windshield, which device includes an emitter for transmitting energy, a sensor for receiving energy, an energy absorbing member and a controller for monitoring energy.
U.S. Pat. No. 6,311,005 describes a sensor device for determining the degree of wetting and/or soiling of a pane in a motor vehicle. The sensor device is said to detect moisture on the outer side of the pane via an optical beam which is arranged in the area of the pane. The sensor device includes a reflector positioned in the pane, the reflector intended to direct the beam through the pane under conditions of total reflection or reflection at the outer side of the pane and at the reflector. The pane additionally includes a light filter which absorbs a selected wavelength of sunlight. Attenuation of the light is said to be reduced via an optically more absorbent layer of the light filter.
U.S. Pat. No. 7,230,260 describes a raindrop sensor provided on a first surface of a transparent body for sensing water attached to a second surface of the transparent body, the raindrop sensor including a light emitting element, a light guide body, a light receiving element and an abnormality determining device. The functions of these various components is also described.
A further issue with prior art rain sensors is that they are subject to misalignment. It was shown in the '183 patent that a rain sensor may be made with segmented lenses. Such lenses, known also as Fresnel lenses, offer compact size. Like conventional lenses, they share a common focal point and focal power across the surface of, for example, a glass pane. Thus, if the optical device is misaligned, or the mounting of the rain sensor induces too much deviation to the optical path, it may be possible for a significant portion of emitter rays to miss the detector lens. The effect is controllable in the sensor of the '183 patent, but there remains room for improvement.
Conventional surface mount emitters of infrared radiation radiate into a hemisphere, more strongly on-axis, and decreasing off-axis. The strength of the radiation is, generally, decreased by as much as 50% at 60 degrees off-axis, and decreases even more rapidly when greater than 60 degrees off-axis. Thus, surface mount emitters, are typically said to have a 60 degree half-angle, or 120 degree cone angle. This angle, times the surface area of the emitter, may be a thought of as the “extent” of the emitter. Similarly, the sensitivity of a surface mount detector of infrared radiation, for example, a photodiode drops rapidly beyond 60 degrees off-axis, and are usually also specified as having a 120 degree acceptance cone.
These emitter and detector extents are each in three-dimensions. So, in examining the figures of the present application, one must realize that most of the rays emitted from the emitters are not in the plane of the page on which the figures is printed. The emitters additionally radiate into and out of the plane of each page, and in most known rain sensors this radiation is not utilized. Even if an emitter captures light rays over a cone angle of 40 degrees, taken in three dimensions, this in total utilizes only 12% of the available angular extent of the emitter. Similarly, a cone angle of 40 degrees for the detector side, utilizes but 12% of the available acceptance angle of the detector. The mechanical and optical constraints of rain sensor design make it very difficult to utilize a high extent. Even the best of known rain sensors would approach using only 25% of the available extent on either the emitter of detector side optics. More typical extent utilization in rain sensors is well under 10%
The rain sensor disclosed in the '183 patent seeks to use as many of the light rays that emanate from the emitter as possible. Only those rays that are ultimately coupled into the detector are of value in sensing rain. Similarly, it is desirable to use all of the available angles to direct rays into the detector. The product of detector area and angle is known in the field of optical engineering as the “extent” (E) of the detector, and good use of the available extent allows the least expensive detector for the purpose. While the sensor of the '183 patent represented an advance over the art that came before, it will be appreciated that there is room for improvement in the utilization of emitter rays and target extent.
A sensor utilizing multiple passes, or deflections, through the windshield, may readily be constructed for windshields with high infrared transmittance. Most modern passenger car windshields, however, strongly absorb infrared light, rendering the multi-pass approach inoperative. It would be better if a rain sensor could work as a single-pass sensor when deployed on infrared absorbing glass, and still function as a multi-pass sensor when deployed on clear glass which allows transmittance of fairly high percentages of infrared radiation.